Intro to Javascript

1 Intro to Javascript

CS380 Client Side Scripting

CS380 Why use client-side programming? 3 PHP already allows us to create dynamic web pages. Why also use client-side scripting?

client-side scripting (JavaScript) benefits: usability : can modify a page without having to post back to the server (faster UI) efficiency : can make small, quick changes to page without waiting for server event-driven : can respond to user actions like clicks and key presses

CS380 Why use client-side programming? 4

server-side programming (PHP) benefits: security : has access to server's private data; client can't see source code compatibility : not subject to browser compatibility issues power : can write files, open connections to servers, connect to databases, ...

CS380 What is Javascript?

a lightweight programming language ("scripting language") used to make web pages interactive insert dynamic text into HTML (ex: user name) react to events (ex: page load user click) get information about a user's computer (ex: browser type) perform calculations on user's computer (ex: form validation)

CS380 What is Javascript?

a web standard (but not supported identically by all browsers)

NOT related to Java other than by name and some syntactic similarities

CS380 Javascript vs Java

interpreted, not compiled

more relaxed syntax and rules fewer and "looser" data types variables don't need to be declared errors often silent (few exceptions)

key construct is the function rather than the class "first-class" functions are used in many situations

contained within a web page and integrates with its HTML/CSS content CS380 Javascript vs Java

+ =

CS380 JavaScript vs. PHP

similarities: both are interpreted, not compiled both are relaxed about syntax, rules, and types both are case-sensitive both have built-in regular expressions for powerful text processing

CS380 JavaScript vs. PHP

differences: JS is more object-oriented: noun.verb(), less procedural: verb(noun) JS focuses on user interfaces and interacting with a document; PHP is geared toward HTML output and file/form processing JS code runs on the client's browser; PHP code runs on the web server

JS <3 CS380 Linking to a JavaScript file: script 11

HTML

script tag should be placed in HTML page's head

script code is stored in a separate .js file

JS code can be placed directly in the HTML file's body or head (like CSS) but this is bad style (should separate content, presentation, and behavior

CS380 Event-driven programming

split breaks apart a string into an array using a delimiter can also be used with regular expressions (seen later)

join merges an array into a single string, placing a delimiter between them

CS380 A JavaScript statement: alert

13 alert("IE6 detected. Suck-mode enabled."); JS

a JS command that pops up a dialog box with a message

CS380 Event-driven programming

you are used to programs start with a main method (or implicit main like in PHP)

JavaScript programs instead wait for user actions called events and respond to them

event-driven programming: writing programs driven by user events

Let's write a page with a clickable button that pops up a "Hello, World" window...

CS380 Buttons

15 HTML

button's text appears inside tag; can also contain images

To make a responsive button or other UI control: 1. choose the control (e.g. button) and event (e.g. mouse 1. click) of interest 2. write a JavaScript function to run when the event occurs 3. attach the function to the event on the control CS380 JavaScript functions

16 function name() { statement ; statement ; ... statement ; } JS function myFunction() { alert("Hello!"); alert("How are you?"); } JS

the above could be the contents of example.js linked to our HTML page

statements placed into functions can be evaluated in response to user events CS380 Event handlers

17 ... HTML

HTML

JavaScript functions can be set as event handlers when you interact with the element, the function will execute

onclick is just one of many event HTML attributes we'll use

but popping up an alert window is disruptive and

CS380annoying A better user experience would be to have the Document Object Model (DOM)

most JS code manipulates elements on an HTML page

we can examine elements' state e.g. see whether a box is checked

we can change state e.g. insert some new text into a div

we can change styles e.g. make a paragraph red DOM element objects

19 Accessing elements: document.getElementById 20 var name = document.getElementById("id"); JS

replace me HTML

function changeText() { var span = document.getElementById("output"); var textBox = document.getElementById("textbox");

textbox.style.color = "red";

} JS

CS380 Accessing elements: document.getElementById 21 document.getElementById returns the DOM object for an element with a given id

can change the text inside most elements by setting the innerHTML property

can change the text in form controls by setting the value property

CS380 Changing element style: element.style 22

CS380 Preetify

function changeText() { //grab or initialize text here

// font styles added by JS: text.style.fontSize = "13pt"; text.style.fontFamily = "Comic Sans MS"; text.style.color = "red"; // or pink? } JS

CS380 24 More Javascript Syntax

CS380 Variables

25 var name = expression; JS

var clientName = "Connie Client"; var age = 32; var weight = 127.4; JS

variables are declared with the var keyword (case sensitive)

types are not specified, but JS does have types ("loosely typed") Number, Boolean, String, Array, Object, Function, Null, Undefined can find out a variable's type by calling typeof CS380 Number type

26 var enrollment = 99; var medianGrade = 2.8; var credits = 5 + 4 + (2 * 3); JS

integers and real numbers are the same type (no int vs. double)

same operators: + - * / % ++ -- = += -= *= /= %=

similar precedence to Java

many operators auto-convert types: "2" * 3 is 6

CS380 Comments (same as Java)

27 // single-line comment /* multi-line comment */ JS

identical to Java's comment syntax

recall: 4 comment syntaxes HTML: CSS/JS/PHP: /* comment */ Java/JS/PHP: // comment PHP: # comment

CS380 Math object

28 var rand1to10 = Math.floor(Math.random() * 10 + 1); var three = Math.floor (Math.PI ); JS

methods: abs, ceil, cos, floor, log, max, min, pow, random, round, sin, sqrt, tan

properties: E, PI

CS380 Special values: null and undefined 29 var ned = null; var benson = 9; // at this point in the code, // ned is null // benson's 9 // caroline is undefined JS

undefined : has not been declared, does not exist

null : exists, but was specifically assigned an empty or null value

Why does JavaScript have both of these? CS380 Logical operators

> < >= <= && || ! == != === !==

most logical operators automatically convert types: 5 < "7" is true 42 == 42.0 is true "5.0" == 5 is true

=== and !== are strict equality tests; checks both type and value "5.0" === 5 is false

CS380 if/else statement (same as Java) 31 if (condition) { statements; } else if (condition) { statements; } else { statements; } JS

identical structure to Java's if/else statement

JavaScript allows almost anything as a condition

CS380 Boolean type

32 var iLike190M = true; var ieIsGood = "IE6" > 0; // false if ("web devevelopment is great") { /* true */ } if (0) { /* false */ } JS

any value can be used as a Boolean "falsey" values: 0, 0.0, NaN, "", null, and undefined "truthy" values: anything else

converting a value into a Boolean explicitly: var boolValue = Boolean(otherValue); var boolValue = !!(otherValue); CS380 for loop (same as Java)

33 var sum = 0; for (var i = 0; i < 100; i++) { sum = sum + i; } JS

var s1 = "hello"; var s2 = ""; for (var i = 0; i < s.length; i++) { s2 += s1.charAt(i) + s1.charAt(i); } // s2 stores "hheelllloo" JS

CS380 while loops (same as Java)

34 while (condition) { statements; } JS

do { statements; } while (condition); JS

break and continue keywords also behave as in Java

CS380 Popup boxes

35 alert("message"); // message confirm("message"); // returns true or false prompt("message"); // returns user input string JS

CS380 Arrays

36 var name = []; // empty array var name = [value, value, ..., value]; // pre-filled name[index] = value; // store element JS

var ducks = ["Huey", "Dewey", "Louie"]; var stooges = []; // stooges.length is 0 stooges[0] = "Larry"; // stooges.length is 1 stooges[1] = "Moe"; // stooges.length is 2 stooges[4] = "Curly"; // stooges.length is 5 stooges[4] = "Shemp"; // stooges.length is 5 JS

CS380 Array methods

37 var a = ["Stef", "Jason"]; // Stef, Jason a.push("Brian"); // Stef, Jason, Brian a.unshift("Kelly"); // Kelly, Stef, Jason, Brian a.pop(); // Kelly, Stef, Jason a.shift(); // Stef, Jason a.sort(); // Jason, Stef JS array serves as many data structures: list, queue, stack, ...

methods: concat, join, pop, push, reverse, shift, slice, sort, splice, toString, unshift push and pop add / remove from back unshift and shift add / remove from front shift and pop return the element that is removed String type

38 var s = "Connie Client"; var fName = s.substring(0, s.indexOf(" ")); // "Connie" var len = s.length; // 13 var s2 = 'Melvin Merchant'; JS methods: charAt, charCodeAt, fromCharCode, indexOf, lastIndexOf, replace, split, substring, toLowerCase, toUpperCase

charAt returns a one-letter String (there is no char type)

length property (not a method as in Java)

Strings can be specified with "" or ''

concatenation with + : 1 + 1 is 2, but "1" + 1 is "11" More about String

39 escape sequences behave as in Java: \' \" \& \n \t \\

converting between numbers and Strings: var count = 10; var s1 = "" + count; // "10" var s2 = count + " bananas, ah ah ah!"; // "10 bananas, ah ah ah!" var n1 = parseInt("42 is the answer"); // 42 var n2 = parseFloat("booyah"); // NaN JS

accessing the letters of a String: var firstLetter = s[0]; // fails in IE var firstLetter = s.charAt(0); // does work in IE var lastLetter = s.charAt(s.length - 1); JS

CS380 Splitting strings: split and join

40 var s = "the quick brown fox"; var a = s.split(" "); // ["the", "quick", "brown", "fox"] a.reverse(); // ["fox", "brown", "quick", "the"] s = a.join("!"); // "fox!brown!quick!the" JS

split breaks apart a string into an array using a delimiter can also be used with regular expressions (seen later)

join merges an array into a single string, placing a delimiter between them